SEISMIC INVESTIGATION OF THE MORPHOLOGY OF A TUNNEL CHANNEL OF THE GREEN BAY LOBE, WISCONSIN, USA

Abstract

Records of subglacial drainage features of previous ice sheets, such as tunnel channels (TCs), provide insight into drainage mechanisms of modern ice sheets. More than 60 tunnel channels were identified in the formerly glaciated landscape of Wisconsin, USA, in the footprint of the Green Bay Lobe of the Laurentide Ice Sheet. I used a combination of a reflection-seismic survey and a hydraulic-potential model to reveal the geometry of a TC and attempt to further understand the dynamics of TC formation. The seismic cross section along the Plainfield tunnel channel (PTC), 16 km up-ice from the terminal margin, shows an absence of a channelized feature in the subsurface. Therefore, the Plainfield tunnel channel likely initiates between 7-16 km up-ice from its terminus in a subglacial setting where the water-flow velocity may have increased, as dictated by the regional bed slope. Relatively fast subglacial water flow would have caused erosion at the ice-bed interface, whereas slower velocities produced little to no erosion of the underlying sediments. The hydraulic-potential model suggests a total area of ~107 km2 for potential subglacial lakes up-ice from the PTC, although this estimate is not sufficient to account for the volume of water needed to form the channel. However, when water does pool in subglacial lakes, permafrost in the area helps to seal in large amounts of water at the ice-bed interface. Additionally, an increase in traction at the bed after a single discharge event likely facilitates supraglacial lake formation. The supraglacial water sources subsequently drain into the subglacial system, which can cause successive subglacial drainage events to occur. The results from the seismic cross-section and the hydraulic-potential model, along with inferences from previous studies, indicate that the PTC is likely formed over several drainage events and had a substantial influence from moulin drainage of supraglacial water in addition to the water stored in subglacial lakes. The Laurentide Ice Sheet was able to store large amounts of subglacial water in this region due to the combination of an adverse bed slope, as well as expansive permafrost during the time of TC formation. This likely means that modern ice sheets, such as along the Western edge of the Greenland Ice Sheet or Thwaites Glacier in West Antarctica, could exhibit similar subglacial and supraglacial drainage behavior in areas with adverse bed slopes similar to this portion of the Laurentide Ice Sheet.